Cesium is a metal at the bottom of the group 1 elements (alkaline metals). It is incredibly reactive in its native metal form. In fact, it cannot be found in nature as its native metal due to this reactivity. Solid cesium metal reacts with water according to the equation below.

2Cs (s) + 2H2O (l) ---> 2CsOH (aq) + H2 (g)

The solid cesium metal reacts with liquid water to produce cesium hydroxide and hydrogen gas. The cesium hydroxide is an ionic solid and readily dissolves in water. It is a metal hydroxide and acts as a base in water. This reaction is incredibly violent and exothermic, meaning it gives off heat. So the heat then ignites the hydrogen gas produced to produce an explosion. This is what makes the reaction so dangerous.

This is classified as a redox reaction. The cesium is readily oxidized to the cesium cation (it loses an electron). One of the hydrogens in water is reduced by this electron. Two of these reduced hydrogens combine to produce hydrogen gas.